1. Field of the Invention
The present invention relates to a cold forging method suitable for forming such gears as a pinion gear and a helical gear.
2. Description of the Prior Art
In such gears as a pinion gear and a helical gear, their tooth shapes are complicated, and in case of producing them by machining, the machining time is long and the cost of machining is high. For this reason, cold forging has heretofore been adopted. In the case of a helical pinion gear constituted by a cylindrical portion of a large diameter with a gear portion formed thereon, it is difficult to form a base part of the gear portion with a high accuracy, so for improving the accuracy of the base portion there heretofore has been given a consideration so as to form a built-up part at the base of the gear portion to improve the flow of a stock, as is disclosed in Japanese Published Unexamined Patent Application Nos. hei 7-308729 and hei 7-310807. Also in Japanese Published Unexamined Patent Application No. hei 11-10274 there is disclosed a technique wherein an expanded part is formed at a base of a gear portion to enhance the accuracy of the tooth surface.
Helical pinion gears are usually produced by cutting or forging. A cold forging method so far adopted commonly will now be described with reference to FIGS. 7 to 9. First, a forming section 4 comprising a cylindrical forming portion 2 and a tooth pattern portion 3 is formed in a die 1, then a cylindrical stock 5 is inserted into the cylindrical forming portion 2 in the forming section 4, a knock-out 6 is positioned at an end of the tooth pattern portion 3 in the forming section 4, and on the side opposite to the knock-out 6 an end of the stock 5 is pressed down with a punch 7. Consequently, as shown in FIG. 7, the stock 5 shifts within the forming section 4 and is formed as such a helical pinion gear 8 as shown in FIG. 8 in the forming section 4 within the die 1. In this case, a space 9 is left between the helical pinion gear 8 and the knock-out 6. In this state, by rotating and pushing up the knock-out 6 the helical gear 8 is taken out upward from the die 1.
The helical pinion gear 8 thus formed by cold forging is made up of a cylindrical portion 10 and a gear portion 11, but there. is the first problem that underfill is apt to occur at the connection between the cylindrical portion 10 and the gear portion 11, that is, at a base part 12 of the gear, portion 11. And it is the second problem that a tip end 13 of the gear portion 11 droops.
For solving the-first problem that underfill is apt to occur at the base part 12 of the gear portion 11, a consideration is given in the foregoing Japanese Published Unexamined Patent Applications hei 7-308729 and hei 7-310807 such that the base 12 of the gear portion 11 is formed with a built-up portion to improve the flow of a stock. Also in the foregoing Japanese Published Unexamined Patent Application hei 11-10274 a measure is taken such that an expanded portion is formed at the base 12 of the gear portion 11 to enhance the accuracy of the tooth surface. However, the presence of such a built-up portion or an expanded portion at the base 12 of the gear portion 11 diminishes a substantially effective tooth surface area.
For solving the second problem that the tip end 13 of the gear portion 11 droops, there has heretofore been adopted such a measure as cutting the tip end 13 by machining and not using it. Thus, the portion once formed cannot be utilized effectively. This means that a fruitless forming has been done.
The present invention has been accomplished in view of the above-mentioned circumstances and it is an object of the invention to provide a cold forging method capable of enhancing the forming accuracy of each component portion and capable of affording a product the use of which requires only a slight finishing work.
In the present invention, a stock inserted into a die is sandwiched between a punch and a knock-out and a predetermined pressing force is applied to the knock-out, while a pressing force larger than the sum of the pressing force of the knock-out and a pressing force for forming is applied to the punch, to effect forming. Thus, the stock is sandwiched between the punch and the knock-out and is subjected to forming while undergoing a large compressive force, so that the forming is carried out in accurate conformity with a pattern shape of the die, whereby there can be obtained a highly accurate product.
In the present invention, the punch and the knock-out are rotated while forming a helical pinion gear, so that the forming of even such a helical pinion gear as is complicated in shape can be done without any problem.
In the present invention, the stock inserted into the die is sandwiched between the punch and the knock-out under a predetermined pressing force, and in this sandwiched state of the stock an axial pressing force larger than the sum of a pressing force of the knock-out and a pressing force for forming is applied to the punch while allowing the punch to rotate. Thus, the stock moves axially while being sandwiched between the punch and the knock-out and while undergoing a large compressive force, and the punch rotates during the forming work. Consequently, forming is carried out in accurate conformity with a pattern shape of the die even if the helix angle of the gear portion is large, and it is possible to obtain a highly accurate product.
In the present invention, the knock-out is mounted so as to rotate about the axis thereof. Therefore, a rotational force created by the punch in the forming work becomes free of resistance even on the knock-out side, whereby the forming of even such a helical pinion gear as is complicated in shape can be done without any problem.
The cold forging apparatus of the present invention comprises a die having a forming section to be formed, a knock-out which is fitted in the die rotatably and which is adapted to move axially, a punch which is fitted in the die rotatably and which is adapted to move axially, and a rotational force imparting means for imparting a rotational force to the punch.
In the present invention, a punch sleeve is fitted in a punch case rotatably, a punch is fitted in the punch sleeve in a rotational direction fixed manner, and a rack is brought into mesh with a gear formed on the punch sleeve to constitute the rotational force imparting means.
In the present invention, a special shape portion adapted to bite in a stock is formed at a tip end of the punch.
In the present invention, the knock-out is held by a thrust bearing.